The broad objective of this research program is to understand mechanisms of DNA synthesis in mammalian cells; the main approach in the initial phase of the program has been investigation of DNA synthesizing enzymes in the mouse plasmacytoma, MOPC-104E. 1. To purify and characterize the various forms of MOPC-104E DNA polymerase. 2. To determine the effect of polyoma virus infection on the amounts of DNA polymerase activities in mouse embryo cells, using methods we have developed for fractionation and detection of the various DNA polymerases. 3. To investigate the replication of polyoma virus DNA using an in vitro system consisting of purified proteins and DNA. 4. To continue the investigation of oncornavirus expression, and DNA polymerase expression in human x mouse hybrid cells. 5. To purify and characterize the mouse intracisternal A-particle-associated DNA polymerase.